Using FIG. 4, one example of conventional pattern forming process for an insulating film formed on a silicon substrate will be described.
As shown in FIG. 4A, after an insulating film 12 such as silicon oxide and/or silicon nitride film is allowed to grow on the silicon substrate 11, a predetermined portion on the insulating film 12 is covered with a photoresist 13.
Then, using a gas mixture of carbon and fluorine, e.g., comprising CF4 and CHF3, as an etching gas, dry etching is applied, thus removing a portion 14 not covered with the photoresist 13 of the insulating film 12.
During the dry etching, a fluorocarbon film 16, a polymer film containing carbon and fluorine, is deposited on the surface of a to-be-treated substrate 15 as shown in FIG. 4B.
Thereafter, the photoresist 13 and the fluorocarbon film 16 is removed by ashing using oxygen gas. At that time, to accelerate the ashing rate, generally, the temperature of the to-be-treated substrate 15 is controlled to be high between 150.degree. C. to 250.degree. C.
As a representative semiconductor device using a side wall film, MOS transistors having a Lightly-Doped Drain (hereinafter, abbreviated as LDD) structure are well known.
One example of conventional LDD formation process will be described referring to FIG. 5.
As shown in FIG. 5A, a gate electrode 18 is formed on the semiconductor silicon substrate 16 via a gate oxide film 17 by the publicly-known photolithography and dry etching techniques and low-concentration diffused layers 19 and 20 serving as part of source and drain are formed with the gate electrode 18 used as mask by means of ion implantation in a self-alignment manner.
Next, as shown in FIG. 5B, a silicon oxide film 22 is allowed to grow in such a manner as to cover the surface of the to-be-treated substrate 21 by the publicly-known CVD technique. Then, using an etching gas containing carbon and fluorine, e.g., a gas mixture of CF4 AND CHF3, as shown in FIG. 5C, an anisotropic dry etching is practiced all over the silicon oxide film 22 in such a manner as to leave only the side face of the gate electrode 18 and thus a side wall film 23 is formed on the side face of the gate electrode 18.
After the formation of the side wall film 23 on the side face of the gate electrode 18, with the gate electrode 18 and the side wall film 23 employed as mask as shown in FIG. 5E, high-concentration diffused layers 24 and 25 serving as the other part of the source and drain are formed in the self-alignment manner by means of ion implantation.
In the process step of forming a MOS transistor of such an LDD structure, as shown in FIG. 5C, a fluorocarbon film 26, a polymer film containing carbon and fluorine, is deposited on the surface of the to-be-treated substrate 21 during the dry etching of the silicon oxide film 22.
For this reason, before the implementation of FIG. 5E after the completion of the process step shown in FIG. 5C, the post-treatment step of removing the fluorocarbon film 26 is carried out as shown in FIG. 5D.
To be specific, from a conventional thinking that the fluorocarbon film 26 can be stripped off by an oxygen plasma method and the semiconductor silicon substrate 16 is not stripped off by the oxygen plasma method, an ordinary oxygen plasma treatment device used for the ashing of a photoresist is employed.
This ordinary oxygen plasma treatment device used for the ashing of a photoresist is so arranged as to control the temperature of a to-be-treated substrate 21 at high temperatures of 150.degree. C. to 250.degree. C., and to remove the photoresist by stripping in an oxygen plasma.
Heretobefore, a description is made of the process step for forming a conventional LDD forming step, but there is another semiconductor device using a side wall film such as MOS transistor with a side wall film utilized for an element separation insulating film.
FIG. 6 shows a conventional example of the process step for forming an element separation insulating film by utilizing a side wall film.
As shown in FIG. 6A, an insulating film 28, constituent part of the insulating film separating a transistor activated region 27 on the semiconductor silicon substrate 16, is formed using publicly-known photolithography and dry etching techniques.
Next, as shown in FIG. 6B, a silicon oxide film 22 is allowed to grow in such a manner as to cover the surface of the to-be-treated substrate 21 by a publicly-known CVD technique. Then, using an etching gas containing carbon and fluorine, e.g., a gas mixture of CF4 AND CHF3, as shown in FIG. 6C, an anisotropic dry etching is practiced all over the silicon oxide film 22 in such a manner as to leave only the side face of the gate electrode 28, and thus a side wall film 23 is formed on the side face of the gate electrode 28.
Thereafter, on the transistor activated region 27 via the gate oxide film 17, as shown in FIG. 6E, a gate electrode 18 is formed using the publicly-known photolithography and dry etching techniques to thus prepare a MOS transistor.
In such a process step of forming a MOS transistor on the element separation insulating film by utilizing a side wall film, as shown in FIG. 6C, a fluorocarbon film 26, or a polymer film containing carbon and fluorine, is deposited on the surface of the to-be-treated substrate 21 during the dry etching of the silicon oxide film 22.
For this reason, before the implementation of FIG. 6E after the completion of the process step shown in FIG. 6C, the post-treatment step for removing the fluorocarbon film 26 to implement an aspect as shown in FIG. 2D becomes necessary. Thus, like the ashing of a fluorocarbon film 26 in the process step of forming a MOS transistor of an LDD structure, an ordinary oxygen plasma treatment device used for the ashing of a photoresist is employed while controlling the temperature of a to-be-treated substrate 21 at high temperatures of 150.degree. C. to 250.degree. C., thereby to accomplish the ashing treatment in an oxygen plasma.